Mycobacterium tuberculosis (Mtb) is responsible for about two million deaths worldwide each year with approximately ten million new cases annually. Like several other intracellular pathogens, the preferred niche for this organism throughout most of the infection, is a modified phagosome-like compartment of the host macrophage. While there are clearly advantages to this pathogenic strategy, growth within a host-derived membrane also poses many challenges. Perhaps the most fundamental is the acquisition of nutrients. How Mtb, or any other vacuolar pathogen, obtains nutrients in this niche is unclear. Furthermore, the environment found inside the phagosome is not static, but rather changes as the disease progresses. Infection with Mtb can be divided into at least two distinct stages: the early preimmune/acute phase and the late postimmune/chronic phase. Previously we used a global genetic interaction mapping strategy to characterize a cholesterol uptake system, called Mce4. This transporter is only required for survival during the chronic stages of infection, suggesting that the nutrients available to the bacterium change as disease progresses. The carbon source(s) used in the acute phase remains unclear. Using a high-throughput screen for transposon mutants that are defective for growth in acute infection, we have found two putative carbohydrate uptake systems and a hexose kinase that are critical for growth during this phase. The putative sugar importers sugABC, rv2038c-2040c and the hexose kinase ppgK, are important for survival during the early stages of infection suggesting that carbohydrates may serve as important nutrients during this phase. We propose a model in which the adaptive immune response alters the compartment in which Mtb resides, and the bacterium adjusts by shifting from carbohydrate to a cholesterol-based metabolism. To test this hypothesis, we propose to characterize these carbohydrate transporters and hexose kinase, identify the host substrates that serve as nutrients, and identify new therapeutic targets. These studies will provide valuable new drug targets and insight regarding how pathogens adapt in nutrient limited environments inside the host.